The invention relates to a low-pressure mercury-vapor discharge lamp comprising a discharge vessel,
which discharge vessel encloses a discharge space containing a filling of mercury and an inert gas in a gastight manner,
electrodes being arranged in the discharge space for generating and maintaining a discharge in said discharge space,
and an electrode shield at least substantially surrounding at least one of the electrodes.
In mercury-vapor discharge lamps, mercury is the primary component for (efficiently) generating ultraviolet (UV) light. An inner surface of the discharge vessel may be provided with a luminescent layer containing a luminescent material (for example a fluorescent powder) for converting UV to other wavelengths, for example to UV-B and UV-A for tanning purposes (sunbed lamps) or to visible radiation. Such discharge lamps are therefore also referred to as fluorescent lamps.
A low-pressure mercury-vapor discharge lamp of the type mentioned in the opening paragraph is known from DE-A 1 060 991. In said known lamp, the electrode shield surrounding the electrode is made from thin sheet titanium. By using an electrode shield, which is also referred to as anode shield or cathode shield, blackening at an inner surface of the discharge vessel is counteracted. In this respect, titanium serves as the getter for chemically binding oxygen, nitrogen and/or carbon.
A drawback of the use of such an electrode shield is that the titanium in the electrode shield may amalgamate with the mercury present in the lamp and, thus, absorb mercury. As a result, the known lamp requires a relatively high dose of mercury to obtain a sufficiently long service life. Injudicious processing of the known lamp after its service life has ended adversely affects the environment.
It is an object of the invention to provide a low-pressure mercury-vapor discharge lamp of the type mentioned in the opening paragraph, which has a relatively low mercury consumption.
To achieve this, the low-pressure mercury-vapor discharge lamp in accordance with the invention is characterized in that, during nominal operation, the temperature of the electrode shield is above 450xc2x0 C.
In the description and the claims of the current invention, the designation xe2x80x9cnominal operationxe2x80x9d is used to indicate operating conditions where the mercury vapor pressure is such that the radiant efficacy of the lamp is at least 80% of that during optimum operation, i.e. operating conditions where the mercury vapor pressure is optimal.
For the proper operation of low-pressure mercury-vapor discharge lamps, the electrodes of such discharge lamps include an (emitter) material having a low so-called work function (reduction of the work function voltage) for supplying electrons to the discharge (cathode function) and receiving electrons from the discharge (anode function). Known materials having a low work function are, for example, barium (Ba), strontium (Sr) and calcium (Ca). It has been observed that, during operation of low-pressure mercury-vapor discharge lamps, material (barium and strontium) of the electrode(s) is subject to evaporation. It has been found that, in general, the emitter material is deposited on the inner surface of the discharge vessel. It has further been found that Ba (and Sr) which is deposited elsewhere in the discharge vessel, no longer participates in the electron emission process. The deposited (emitter) material further forms mercury-containing amalgams on the inner surface, as a result of which the quantity of mercury available for the discharge decreases (gradually), which may adversely affect the service life of the lamp. In order to compensate for such a loss of mercury during the service life of the lamp, a relatively high dose of mercury in the lamp is necessary, which is undesirable from the point of view of environmental protection.
The provision of an electrode shield, which surrounds the electrode(s) and, during nominal operation, is at a temperature above 250xc2x0 C., causes the reactivity of materials in the electrode shield relative to the mercury present in the discharge vessel, leading to the formation of amalgams (Hgxe2x80x94Ba, Hgxe2x80x94Sr), to be reduced.
It has further been found in experiments that emitter material which evaporates from the electrode reacts with the material of the electrode shield, thereby forming oxides (BaO or SrO). During (nominal) operation of the discharge lamp, mercury makes a bond with these oxides of evaporated emitter material. If reactive oxygen is present in the proximity of the electrode, then BaO, SrO and/or HgO and, possibly, SrHgO2 and BaHgO2 are formed. If, in addition, tungsten (originating from the electrode) is deposited (in the case of a cold start, tungsten is sputtered) also WOX and HgWOX are formed. Without being obliged to give any theoretical explanation, it seems that although BaO and SrO do not react with mercury under normal thermal conditions, the presence of the discharge in the discharge space plays a part in the formation of these compounds of mercury and the oxides of evaporated emitter material. At temperatures above 450xc2x0 C. the mercury is released again, as a result of dissociation of said compounds of mercury and the oxides of evaporated emitter material, and the released mercury is available again for the discharge. Particularly HgO dissociates at a temperature of 450xc2x0 C. or higher; the compounds SrHgO2 and BaHgO2 are slightly more stable. The inventors have recognized that by using an electrode shield having a temperature of 450xc2x0 C. or higher, mercury is released from the compounds of mercury and oxides of emitter material. A particularly suitable temperature of the electrode shield is approximately 500xc2x0 C., at which temperature also the dissociation of, in particular, SrHgO2 and BaHgO2 takes place relatively rapidly. It cannot be excluded, however, that the stainless steel also acts as a getter (corrosion) at the above-mentioned relatively high temperatures, leading to an additional reduction of the formation of HgO-type compounds.
The known lamp comprises an electrode shield of thin sheet titanium, which material relatively readily amalgamates with mercury. The mercury consumption of the discharge lamp is limited by substantially reducing the degree to which the material of the electrode shield, which surrounds the electrode(s), reacts with mercury and/or bonds with mercury.
In addition, the use of an electrically insulating material precludes the development of short circuits in the electrode wires and/or in a number of windings of the electrode(s). The known lamp has an electrode shield of an electroconductive material, which, in addition, relatively readily forms an amalgam with mercury. The mercury consumption of the discharge lamp is limited by substantially reducing the degree to which the material of the shield surrounding the electrode(s) reacts with mercury.
In order to obtain an electrode shield which can be heated to such high temperatures during nominal operation of the discharge lamp and, during operation, is capable of maintaining said high temperatures throughout the service life of the discharge lamp, the electrode shield is preferably manufactured from a metal or a metal alloy which can withstand temperatures of 450xc2x0 C. or higher. An xe2x80x9celectrode shield which can withstand high temperaturesxe2x80x9d is to be taken to mean in the description of the current invention, that, during the service life of the discharge lamp and at said temperatures, the material from which the electrode shield is manufactured does not show signs of degassing and/or evaporation, which adversely affect the operation of the discharge lamp, and that no appreciable changes in shape occur in the electrode shield at such high temperatures.
A preferred embodiment of the low-pressure mercury-vapor discharge lamp is characterized in accordance with the invention in that the electrode shield is made from stainless steel. Stainless steel is a material which is resistant to high temperatures. Stainless steel has a high corrosion resistance, a relatively low coefficient of thermal conduction and a relatively poor thermal emissivity as compared to the known materials. By virtue thereof it becomes possible to manufacture a stainless steel electrode shield which can relatively readily reach temperatures above 450xc2x0 C. by exposure to heat originating from the electrode. Materials which can very suitably be used to manufacture the electrode shield are chromium-nickel-steel and Duratherm 600.
In a particularly favorable embodiment of the low-pressure mercury-vapor discharge lamp in accordance with the invention, the electrode shield is provided, at a side facing away from the electrode, with a low-emissivity coating for reducing the radiation losses of the electrode shield. By applying such a layer to an outer surface of the electrode shield, the desired relatively high temperatures of the electrode shield can be reached more readily. The low-emissivity coating preferably comprises chromium or a noble metal, for example gold. Other materials which can suitably be used for a low-emissivity coating on the outer surface of the electrode shield are titanium nitride, chromium carbide, aluminum nitride and silicon carbide. In an alternative embodiment of the low-pressure mercury-vapor discharge lamp, the electrode shield is polished on a side facing the discharge. Also a polishing treatment of the outer surface of the electrode shield causes the heat radiation by the electrode shield to be reduced.
A further preferred embodiment of the low-pressure mercury-vapor discharge lamp in accordance with the invention is characterized in that the electrode shield is provided, at a side facing the electrode, with an absorbing coating for absorbing radiation. By applying a layer having a relatively high emissivity in the infrared radiation range, the heat-absorbing capacity of the electrode shield is increased. By virtue thereof, the desired relatively high temperatures of the electrode shield can be reached more readily. The absorbing coating preferably comprises carbon.
The shape of the electrode shield, its position relative to the electrode and the way in which the electrode shield is provided influence the temperature of the electrode shield. Electrodes in low-pressure mercury-vapor discharge lamp are generally elongated and cylindrically symmetric, for example a coil with windings about a longitudinal axis. A tubularly shaped electrode shield harmonizes very well with such a shape of the electrode. Preferably, an axis of symmetry of the electrode shield extends substantially parallel to, or substantially coincides with, the longitudinal axis of the electrode. In the latter case, the average distance from an inside of the electrode shield to an external dimension of the electrode is at least substantially constant.
Preferably, the electrode shield is provided with a slit on a side facing the discharge space. A slit in the electrode shield in the direction of the discharge causes a relatively short discharge path between the electrodes of the low-pressure mercury-vapor discharge lamp. This is favorable for a high efficiency of the lamp. The slit preferably extends parallel to the axis of symmetry of the electrode shield (so-called lateral slit in the electrode shield). In the known lamp, the aperture or slit in the electrode shield faces away from the discharge space.
The electrode shield is generally held in the desired position around the electrode by means of a support wire, which support wire can be mounted in the discharge vessel in various ways. A further preferred embodiment of the low-pressure mercury-vapor discharge lamp in accordance with the invention is characterized in that a support wire carries the electrode shield, and at least a part of said support wire is made from stainless steel. Stainless steel has a relatively low coefficient of thermal conduction, thereby reducing the emission of heat from the electrode shield to the support wire.